In order to transmit an optical WDM signal over long fiber links, amplifier modules are required after individual transmission sections. An effective method for additional amplification of a signal is based on the use of at least one erbium-doped fiber amplifier EDFA which comprise one or more pump sources connected to an erbium-doped fiber EDF, said pump sources having wavelengths of approx. 980 nm and/or 1480 nm for amplification of the WDM signal in the C and L bands (approx. 1525–1565 nm and approx. 1570–1610 nm). An implementation of an erbium-doped fiber amplifier is known from U.S. Pat. No. 5,140,456. In that implementation pump signals from the pump sources are injected into the erbium-doped fiber and enable an inversion or partial population of a higher energy level through absorption which can lead to a gain or loss depending on the degree of inversion and depending on the wavelength range of the WDM signal. The main purpose of an erbium-doped fiber amplifier EDFA is to ensure a flat and high gain with a low noise figure. An example of the implementation of a fiber amplifier of this type is described in detail in “Optical Fiber Amplifier for WDM Optical Networks”, Sun Y., Srivastava A. K., Zhou Jianhui, Sulhoff J. W., Bell Technical Journal, January–March 1999. In order to achieve a low noise figure together with a high gain, for a two-stage amplifier (see FIG. 2) a high inversion is set in the first stage and a small inversion in the second stage (see page 189, 1st column).
Methods for compensating a tilting of the gain for all channels of the WDM signal are also known. A topical example is described in “Active Gain-Slope Compensation of EDFA Using Thulium-Doped Fiber as Saturable Absorber”, Kitabayashi T., Aizawa T., Sakai T., Wada A., IEICE Transactions on Electronics, Vol. E84-C, No. 5, pp. 605–609, May 2001. In this case, with the use of cascaded thulium-doped and erbium-doped fibers the positive and negative gain tilts are compensated in the C band. A requirement for this is the use of the thulium doped fibers in the saturation range in the case of a transmission in the C band.
Also known from EP 1 045 535 A2 is a means of control for obtaining a flat gain for all channels of a transmitted WDM signal, in particular in the event of temperature changes (see Abstract).
The means of control comprises among other things a pump device with two pump sources disposed distributed over an erbium-doped fiber EDF for amplifying a transmitted WDM signal whose power at the end of the transmission section is determined by means of a photodiode and controlled in such a way that corresponding output powers of the pump sources are regulated by means of a control circuit in order to achieve a wavelength-independent gain for all channels of the WDM signal at the end of the transmission section. Two further photodiodes and a variable optical attenuation element controlled by a controller are provided in order to vary the gain of the transmitted channels.
The current amplifiers for WDM signals are therefore implemented in such a way as to deliver optimal noise figures and/or wavelength-independent and high gains at the maximum channel number as signal utilization. Due to the high channel numbers of modern WDM transmission systems, the pumping of the erbium-doped fiber(s) requires very high pump powers which often can only be provided by coupling pump light into the erbium-doped fiber at multiple points within an amplifier stage.
At startup time there are generally only a few channels active simultaneously or the transmission system is already configured such that it can be expanded only up to a certain number of channels. Due to the significantly reduced input and output powers compared to the full expansion, however, a lower pump output must be set. If each of the pump powers is now reduced by a proportion, this may lead to an increase in the noise figure.
The following figure (FIG. 1) describes an optical amplification according to the prior art. Its characteristics are explained and the disadvantages which are eliminated by the present invention described.
In FIG. 1 the noise figure NF is shown as a function of the input power P of the WDM signal. In order to amplify the WDM signal, a pump device with two pump sources disposed over an erbium-doped fiber is provided. The first pump source has a wavelength of 980 nm and the second pump source has wavelengths of 980 nm (curve C1) and 1480 nm (curve C2). It is assumed that corresponding pump signals of the investigated pump device are injected co- and contradirectionally in the input area and in a further area, respectively, of the erbium doped fiber. With this pump device, each of the pump powers of the pump sources is reduced or increased in equal proportions depending on the change in the input power of the WDM signal, i.e. also depending on the change in the number of active channels. With small input powers, the noise figure increases.
DE 100 58 059 A1 discloses an optical amplifier for WDM signals which comprises a replaceable fiber module for adjusting the amplification to different input levelswhile achieving optimal noise performance.
In order to achieve small noise figures, there is known from the literature source “Quantum Limited Noise Figure Operation of High Gain Erbium Doped Fiber Amplifiers”, Lumbohlt et al., Journal of Lightwave Technology, Vol. 11, No. 8, August 1993, an optical amplifier with a co-, contra- or bidirectionally pumped amplification fiber over which an insulator to suppress the noise from ASE (Amplified Spontaneous Emission) is disposed. Depending on the positioning of the insulator along the amplification fiber, the noise figure is correspondingly adjustable and can therefore be minimized.
A further literature source “Reliable 1.01-μm Band Laser Diode Pumped Praseodynium-Doped In/Ga-Based Fluride Fiber Amplifiers at 1.3 μm”, Isshiki et al., Journal of Lightwave Technology, Vol. 16, No. 12, December 1998 discloses a bidirectionally pumped optical fiber amplifier with a praseodynium-doped In/Ga-based fluoride fiber as the amplification fiber. The levels of an amplified WDM signal at the output of the optical fiber amplifier can be compensated with the aid of a control unit by means of a control of pump laser diodes inserted in the circuit between control unit on the one side and input and output line on the other side. A suitable length of amplification fiber must be chosen in order to achieve high gains of the optical fiber amplifier.
Similarly for controlling the gain there is known from U.S. Pat. No. 5,745,283 a bidirectional pumped fiber amplifier with two laser diodes as pump source for amplification of multiple channels of a WDM signal whose individual pump powers are controlled as a function of a tapped-off proportion of each output line of the channels at the end of the fiber amplifier. In this way differences in gain between the channels are suppressed. A separation of each channel at the end of the fiber amplifier remains complicated and expensive, however. If there are changes to or an expansion of the number of channels, this system does not allow any flexible solution.
Finally, U.S. Pat. No. 6,111,686 discloses an optical amplifier and a method for optical amplification of a signal wherein a reduction in the power consumption as well as in the costs of the system is achieved mainly through the use of a single pump source instead of two pump sources. With known methods, two different modes of operation are provided. On the one hand, in order to reduce the noise figure of the optical amplfier, the output pump power is subdivided into two pump signals for ce and contradirectional pumping such that the power of the first codirectional pump signal is higher than that of the second contradirectional pump signal. On the other hand, in order to achieve a high output power of the optical amplifier, the power of the second pump signal is set to a greater value than that of the first pump signal. Apart from these two settings, namely to achieve a small noise figure on the one hand and to achieve a high output power on the other hand, no further settings are provided.